Boethius, answered from the texts and cited to the page.
Aristotelian categorical analysis can clarify Trinitarian and Christological problems. The categories Boethius applies — substance, accident, person, nature, relation, quantity, quality — provide the technical vocabulary for distinguishing what is at issue in theological controversies that had divided the late-antique Church.1 De Trinitate addresses the question how Father, Son, and Holy Spirit can be one God: the answer involves the distinction between substantial predication (predicating what something is essentially) and relational predication (predicating something said in relation to something else).2
The persons of the Trinity differ relationally (Father in relation to Son, Son in relation to Father) but are one in substance. Contra Eutychen et Nestorium addresses the Christological controversies: Eutyches held that the divine and human natures of Christ are fused into one nature; Nestorius held that they are two persons. The correct position requires a distinction between nature (the essential properties making something what it is) and person (an individual instance of a rational nature).
Christ is one person — one individual substance of a rational nature — but with two natures, divine and human. The famous definition: persona est rationalis naturae individua substantia — person is an individual substance of a rational nature. The De Hebdomadibus develops an axiomatic method anticipating Anselm and the scholastic synthesis: from explicit definitions and self-evident principles, theological conclusions are derived by formal reasoning.
The Tractates are not just abstract theology; they are interventions in active ecclesiastical disputes (the Theopaschite controversy, the Catholic-Arian conflict in Ostrogothic Italy).3
Their meaning is determined by the contingent subject; for some of them denote substance in making predication of other things, others belong to the class of accidents. But when these categories are applied to God they change their meaning entirely. Relation, for instance, cannot be predicated at all of God; for substance in Him is not really substantial but supersubstantial.Consolation of Philosophy and Theological Tractates
But Catholic Christians, allowing no difference of merit in God, assuming Him to be Pure Form and believing Him to be nothing else than His own essence, rightly regard the statement "the Father is God, the Son is God, the Holy Spirit is God, and this Trinity is one God," not as an enumeration of different things but as a reiteration of one and the same thing, like the statement, "blade and brand are one sword" or…Consolation of Philosophy and Theological Tractates
And although the definition which we have already given is enough to prove Nestorius wrong, his error shall be further declared by the following argument.Consolation of Philosophy and Theological Tractates
The Opuscula Sacra were among the most-commented texts of the early Middle Ages and shaped the methodology of scholastic theology. The definition of person became the standard scholastic definition (used by Aquinas, Bonaventure, Scotus, and the broader medieval tradition). The De Hebdomadibus axiomatic method anticipates the monological style of Anselm and the scholastic synthesis. Contemporary engagement runs through personal-identity theory (where Boethius's definition remains a reference point), Trinitarian theology (where the substantial-vs-relational predication distinction continues to do work), and the historical study of late-antique and medieval theology.